The present invention relates in general to a digital signal processing radio receiver wherein an intermediate frequency signal is digitally processed, and more specifically, to filtering the intermediate frequency signal with a filter bandwidth which is adaptively controlled in response to the strength of adjacent or alternate channel interferers relative to the strength of the desired channel.
Commercial AM and FM broadcast bands include a plurality of evenly spaced channels. A particular broadcast station is allocated a unique channel to conduct broadcasting within an assigned frequency range.
The power spectrum of a broadcast transmission consists of the strength of RF signals irradiating a receiving antenna over a range of frequencies. Even though most energy in a transmission can be limited to its assigned channel, efficient use of all channels implies that some radiated energy will be at frequencies outside their assigned channels. Assignment of frequency channels to individual transmitters is determined according to geographic location and other factors to minimize interference between transmissions in adjacent channels and alternate channels (an alternate channel is a channel two channels away from the desired channel of interest). Since demand in populous areas is high for the limited number of channels available, adjacent and/or alternate channel interferers are often present. Therefore, radio receivers must be able to perform adequately in situations where a strong signal on an adjacent or alternate channel creates signal components in the desired channel which interfere with reception of the desired signal. In addition, channels even farther away than an alternate channel and even sources not associated with the broadcast band can become significant interferers.
Prior art radio receivers are known which detect the presence of adjacent channel or other interference in various ways and which attempt to reduce effects of the interference by narrowing the receiver bandwidth (or by shifting the receiver passband for the desired channel). Separate filters or returning of the receiver are typical methods for detecting presence of an interfering channel. The prior art passband switching introduces transient effects which can be heard as noise. Furthermore, analog filtering that has been used does not provide sharp band edges (i.e., they have slow roll-off), so that not all of an interferer is removed.
With the advent of high speed digital signal processing (DSP) components, radio receivers are being introduced using DSP processors to implement demodulation and various other functions in a radio receiver. In particular, it is becoming possible to digitally process the intermediate frequency (IF) signal or even the radio frequency (RF) signal, thus avoiding a large number of analog circuits and components with their associated costs and space requirements. In order to keep costs down for a particular radio receiver, DSP performance in terms of 1) chip area required for processing, and 2) execution time need to be minimized.